Brushless DC motors generally include rotor position detectors, such as Hall sensors, positioned on the motor stator. Such devices sense the magnetic field of permanent magnets on the rotor as such magnets move over the sensors. By detecting the magnitude and polarity of the magnetic field, Hall sensors provide angular position information which is employed to switch current to the proper motor windings at the correct time through a process known as commutation. Such position information is critical for efficient motor start up in the desired direction. Failure to achieve start-up in the proper direction or to minimize reverse rotation of the rotor could cause damage to the heads when utilized in magnetic storage disc drive devices.
Motor construction typically includes a rotor formed of one or more pairs of magnetic pole pieces which rotate relative to a phase stator. Frequently, brushless DC motor construction consists of a three phase stator winding. Individual motor phases can be energized separately or in combination. The energizing of such phase or phases is typically predetermined. The resulting torque developed due to an electrical current in a motor winding is a function of the angular position of the rotor. Generally, the function can be approximated by a sinusoidal curve. Over one electrical cycle, one permanent magnetic pole pair, half of the torque developed is positive and the other half is negative. The magnitude and sign of the developed torque depends on the angular position of the rotor relative to the windings. Without rotor position information, maximum torque and correct rotation direction can not be guaranteed.
The requirement for smaller and lower cost brushless DC motors has required the removal of Hall or similar sensors from the motors. Reverse rotation of DC motors in disc drives can result in head or medium damage. Therefore, alternative methods are necessary for obtaining angular position for motor start up.